The 3GPP Rel-13 feature “License-Assisted Access” (LAA) allows LTE equipment to also operate in the unlicensed 5 GHz radio spectrum. The unlicensed 5 GHz spectrum is used as a complement to the licensed spectrum. An ongoing 3GPP Rel-14 work item adds UL transmissions to LAA. Accordingly, devices such as LTE user equipment (UEs), for example, connect in the licensed spectrum (primary cell or PCell) and use carrier aggregation to benefit from additional transmission capacity in the unlicensed spectrum (secondary cell or SCell). Standalone operation of LTE in unlicensed spectrum is also possible and is under development by the MuLTEfire Alliance.
For the case of standalone LTE-U, the initial random access (RA) and subsequent uplink (UL) transmissions take place entirely on the unlicensed spectrum. Regulatory requirements may not permit transmissions in the unlicensed spectrum without prior channel sensing. Since the unlicensed spectrum must be shared with other radios of similar or dissimilar wireless technologies, a so-called listen-before-talk (LBT) procedure may be used. LBT involves sensing the medium for a pre-defined minimum amount of time and backing off if the channel is busy. Today, the unlicensed 5 GHz spectrum is mainly used by equipment implementing the IEEE 802.11 Wireless Local Area Network (WLAN) standard, also known under its marketing brand as “Wi-Fi.”
FIG. 1 illustrates the basic LTE downlink physical resource. LTE uses Orthogonal Frequency Division Multiplexing (OFDM) in the downlink and Discrete Fourier Transform (DFT)-spread OFDM (also referred to as single-carrier FDMA (SC-FDMA)) in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. The uplink subframe has the same subcarrier spacing as the downlink, and the same number of SC-FDMA symbols in the time domain as OFDM symbols in the downlink.
FIG. 2 illustrates the LTE time-domain structure. In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms as shown in FIG. 2. Each subframe comprises two slots of duration 0.5 ms each, and the slot numbering within a frame ranges from 0 to 19. For normal cyclic prefix, one subframe consists of 14 OFDM symbols. The duration of each symbol is approximately 71.4 μs.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks (RBs), where a RB corresponds to one slot (0.5 ms) in the time domain and twelve contiguous subcarriers in the frequency domain. A pair of two adjacent RBs in time direction (1.0 ms) is known as a resource block pair. RBs are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Downlink transmissions are dynamically scheduled. Specifically, in each subframe the base station transmits control information about which terminals data is transmitted to and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3, or 4 OFDM symbols in each subframe and the number n=1, 2, 3, or 4 is known as the Control Format Indicator (CFI). The downlink subframe also contains common reference symbols, which are known to the receiver and used for coherent demodulation of the control information. A downlink subframe with CFI=3 OFDM symbols as control is illustrated in FIG. 3. The reference symbols shown there are the cell specific reference symbols (CRS) and are used to support multiple functions including fine time and frequency synchronization and channel estimation for certain transmission modes.
Uplink transmissions are dynamically scheduled, i.e., in each downlink subframe the base station transmits control information about which terminals should transmit data to the network node in subsequent subframes, and upon which resource blocks the data is transmitted. The uplink resource grid is comprised of data and uplink control information in the PUSCH, uplink control information in the PUCCH, and various reference signals such as demodulation reference signals (DMRS) and sounding reference signals (SRS). DMRS are used for coherent demodulation of PUSCH and PUCCH data, whereas SRS is not associated with any data or control information but is generally used to estimate the uplink channel quality for purposes of frequency-selective scheduling. An example uplink subframe is shown in FIG. 4. Note that UL DMRS and SRS are time-multiplexed into the UL subframe, and SRS are always transmitted in the last symbol of a normal UL subframe. The PUSCH DMRS is transmitted once every slot for subframes with normal cyclic prefix, and is located in the fourth and eleventh SC-FDMA symbols.
From LTE Rel-11 onwards, DL or UL resource assignments can also be scheduled on the enhanced Physical Downlink Control Channel (EPDCCH). For Rel-8 to Rel-10 only the Physical Downlink Control Channel (PDCCH) is available. Resource grants are UE specific and are indicated by scrambling the DCI Cyclic Redundancy Check (CRC) with the UE-specific C-RNTI identifier.
If a wireless device, which may include a UE, has uplink data waiting for transmission in its buffer but does not have any scheduled UL grants, it can send a 1-bit scheduling request (SR) to the serving or primary cell using available PUCCH resources. The SR can be sent using PUCCH Format 1, or be multiplexed with HARQ ACK/NACK feedback in PUCCH Formats 1a, 1b, or 3. The SR modulation is based on on-off keying where a ‘+1’ indicates the SR, and nothing is sent if SR is not transmitted. The UE-specific SR transmission periodicity and SR subframe offset are configured by higher-layer signaling, as shown in Table 1.
TABLE 1SR periodicity and offset configurations (TS 36.213 v. 12.3.0, Rel-12)SR configuration IndexSR periodicity (ms)SR subframe offsetISRSRPERIODICITYNOFFSET, SR0-45ISR   5-1410ISR-5 15-3420ISR-1535-7440ISR-35 75-15480ISR-75155-1562 ISR-1551571 ISR-157Once an eNB receives a SR, it can send an UL grant to the UE and make additional scheduling decisions based on Buffer Status Reports sent by the UE on PUSCH.
The LTE Rel-10 standard supports bandwidths larger than 20 MHz. One important requirement on LTE Rel-10 is to assure backward compatibility with LTE Rel-8. This should also include spectrum compatibility. That would imply that an LTE Rel-10 carrier, wider than 20 MHz, should appear as a number of LTE carriers to an LTE Rel-8 terminal. Each such carrier can be referred to as a Component Carrier (CC). In particular for early LTE Rel-10 deployments it can be expected that there will be a smaller number of LTE Rel-10-capable terminals compared to many LTE legacy terminals. Therefore, it is necessary to assure an efficient use of a wide carrier also for legacy terminals, i.e. that it is possible to implement carriers where legacy terminals can be scheduled in all parts of the wideband LTE Rel-10 carrier. The straightforward way to obtain this would be by means of Carrier Aggregation (CA). CA implies that an LTE Rel-10 terminal can receive multiple CC, where the CC have, or at least the possibility to have, the same structure as a Rel-8 carrier. CA is illustrated in FIG. 5. A CA-capable UE is assigned a primary cell (PCell) which is always activated, and one or more secondary cells (SCells) which may be activated or deactivated dynamically.
The number of aggregated CC as well as the bandwidth of the individual CC may be different for uplink and downlink. A symmetric configuration refers to the case where the number of CCs in downlink and uplink is the same whereas an asymmetric configuration refers to the case that the number of CCs is different. It is important to note that the number of CCs configured in a cell may be different from the number of CCs seen by a terminal: A terminal may for example support more downlink CCs than uplink CCs, even though the cell is configured with the same number of uplink and downlink CCs.
In typical deployments of WLAN, carrier sense multiple access with collision avoidance (CSMA/CA) is used for medium access. This means that the channel is sensed to perform a clear channel assessment (CCA), and a transmission is initiated only if the channel is declared as Idle. In case the channel is declared as Busy, the transmission is essentially deferred until the channel is deemed to be Idle.
A general illustration of the listen before talk (LBT) mechanism of Wi-Fi is shown in FIG. 6. After a Wi-Fi station A transmits a data frame to a station B, station B shall transmit the ACK frame back to station A with a delay of 16 μs. Such an ACK frame is transmitted by station B without performing a LBT operation. To prevent another station interfering with such an ACK frame transmission, a station shall defer for a duration of 34 μs (referred to as DIFS) after the channel is observed to be occupied before assessing again whether the channel is occupied. Therefore, a station that wishes to transmit first performs a CCA by sensing the medium for a fixed duration DIFS. If the medium is idle then the station assumes that it may take ownership of the medium and begin a frame exchange sequence. If the medium is busy, the station waits for the medium to go idle, defers for DIFS, and waits for a further random backoff period.
Using the LBT protocol, when the medium becomes available, multiple Wi-Fi stations may be ready to transmit, which can result in collision. To reduce collisions, stations intending to transmit select a random backoff counter and defer for that number of slot channel idle times. The random backoff counter is selected as a random integer drawn from a uniform distribution over the interval of [0, CW]. The default size of the random backoff contention window, CWmin, is set in the IEEE specs. Note that collisions can still happen even under this random backoff protocol when there are many stations contending for the channel access. Hence, to avoid recurring collisions, the backoff contention window size CW is doubled whenever the station detects a collision of its transmission up to a limit, CWmax, also set in the IEEE specs. When a station succeeds in a transmission without collision, it resets its random backoff contention window size back to the default value CWmin.
FIG. 7 illustrates licensed-assisted access (LAA) to unlicensed spectrum using LTE carrier aggregation. Up to now, the spectrum used by LTE has been dedicated to LTE. This has the advantage that the LTE system does not need to care about the coexistence issue and the spectrum efficiency can be maximized. However, the spectrum allocated to LTE is limited, and the allocated spectrum cannot meet the ever increasing demand for larger throughput from applications and/or services. Therefore, a new study item has been initiated in 3GPP on extending LTE to exploit unlicensed spectrum in addition to licensed spectrum. Unlicensed spectrum can, by definition, be simultaneously used by multiple different technologies. Therefore, LTE needs to consider the coexistence issue with other systems such as IEEE 802.11 (Wi-Fi). Operating LTE in unlicensed spectrum in the same manner as in licensed spectrum can seriously degrade the performance of Wi-Fi, as Wi-Fi will not transmit once it detects the channel is occupied.
Furthermore, one way to utilize the unlicensed spectrum reliably is to transmit essential control signals and channels on a licensed carrier. That is, as shown in FIG. 7, a UE is connected to a PCell in the licensed band and one or more SCells in the unlicensed band. In this application, a secondary cell in unlicensed spectrum is referred to as a licensed-assisted access secondary cell (LAA SCell).
FIG. 8 illustrates UL LAA listen before talk (LBT). In Rel-13 LAA, LBT for DL data transmissions follow a random backoff procedure similar to that of Wi-Fi, with CW adjustments based on HARQ NACK feedback. Several aspects of UL LBT were discussed during Release 13. With regard to the framework of UL LBT, the discussion focused on the self-scheduling and cross-carrier scheduling scenarios. UL LBT imposes an additional LBT step for UL transmissions with self-scheduling, since the UL grant itself requires a DL LBT by the eNB. The UL LBT maximum CW size should then be limited to a very low value to overcome this drawback, if random backoff is adopted. Therefore, Release 13 LAA recommended that the UL LBT for self-scheduling should use either a single CCA duration of at least 25 μs (similar to DL DRS), or a random backoff scheme with a defer period of 25 μs including a defer duration of 16 us followed by one CCA slot, and a maximum contention window size chosen from X={3, 4, 5, 6, 7}. These options are also applicable for cross-carrier scheduling of UL by another unlicensed SCell. FIG. 8 illustrates an example UL LBT and UL transmission when the UL grant is sent on an unlicensed carrier.
SR transmission opportunities are not guaranteed for MuLTEfire due to LBT requirements and the possibility for any subframe to be used for either UL or DL transmissions. Therefore, a periodic SR opportunity may be blocked due to failed LBT or conflict with a DL transmission from the eNB. There is currently no solution for robust SR transmission and multiplexing in LBT systems with multiple users in general.